MDEV-11573 JSON_LENGTH returns incorrect results.

        Item_func_json_length fixed.

mysql-test/r/func_json.result

@@ -425,6 +425,12 @@ json_length('{}')
 select json_length('[1, 2, {"a": 3}]');
 json_length('[1, 2, {"a": 3}]')
 3
+select json_length('{"a": 1, "b": {"c": 30}}', '$.b');
+json_length('{"a": 1, "b": {"c": 30}}', '$.b')
+1
+select json_length('{"a": 1, "b": {"c": 30}}');
+json_length('{"a": 1, "b": {"c": 30}}')
+2
 create table json (j INT);
 show create table json;
 Table	Create Table

mysql-test/t/func_json.test

@@ -170,6 +170,8 @@ select json_depth('[10, {"a": 20}]');
 select json_length('');
 select json_length('{}');
 select json_length('[1, 2, {"a": 3}]');
+select json_length('{"a": 1, "b": {"c": 30}}', '$.b');
+select json_length('{"a": 1, "b": {"c": 30}}');
 
 create table json (j INT);
 show create table json;

sql/item_jsonfunc.cc

@@ -1599,29 +1599,85 @@ String *Item_func_json_merge::val_str(String *str)
 }
 
 
+void Item_func_json_length::fix_length_and_dec()
+{
+  if (arg_count > 1)
+    path.set_constant_flag(args[1]->const_item());
+}
+
+
 longlong Item_func_json_length::val_int()
 {
   String *js= args[0]->val_str(&tmp_js);
   json_engine_t je;
   uint length= 0;
+  uint array_counters[JSON_DEPTH_LIMIT];
 
   if ((null_value= args[0]->null_value))
     return 0;
 
-
   json_scan_start(&je, js->charset(),(const uchar *) js->ptr(),
                   (const uchar *) js->ptr() + js->length());
 
-  do
+  if (arg_count > 1)
   {
-    if (je.state == JST_VALUE)
+    /* Path specified - let's apply it. */
+    if (!path.parsed)
+    {
+      String *s_p= args[1]->val_str(&tmp_path);
+      if (s_p &&
+          json_path_setup(&path.p, s_p->charset(), (const uchar *) s_p->ptr(),
+                          (const uchar *) s_p->ptr() + s_p->length()))
+      {
+        report_path_error(s_p, &path.p, 2);
+        goto null_return;
+      }
+      path.parsed= path.constant;
+    }
+    if (args[1]->null_value)
+      goto null_return;
+
+    path.cur_step= path.p.steps;
+    if (json_find_path(&je, &path.p, &path.cur_step, array_counters))
+    {
+      if (je.s.error)
+        goto err_return;
+      goto null_return;
+    }
+  }
+  
+
+  if (json_read_value(&je))
+    goto err_return;
+
+  if (json_value_scalar(&je))
+    return 1;
+
+  while (json_scan_next(&je) == 0 &&
+         je.state != JST_OBJ_END && je.state != JST_ARRAY_END)
+  {
+    switch (je.state)
+    {
+    case JST_VALUE:
+    case JST_KEY:
       length++;
-  } while (json_scan_next(&je) == 0);
+      break;
+    case JST_OBJ_START:
+    case JST_ARRAY_START:
+      if (json_skip_level(&je))
+        goto err_return;
+      break;
+    default:
+      break;
+    };
+  }
 
   if (!je.s.error)
-    return length - 1;
+    return length;
 
+err_return:
   report_json_error(js, &je, 0);
+null_return:
   null_value= 1;
   return 0;
 }

sql/item_jsonfunc.h

@@ -67,8 +67,8 @@ class Item_func_json_exists: public Item_int_func
   String tmp_js, tmp_path;
 
 public:
-  Item_func_json_exists(THD *thd, Item *js, Item *path):
-    Item_int_func(thd, js, path) {}
+  Item_func_json_exists(THD *thd, Item *js, Item *i_path):
+    Item_int_func(thd, js, i_path) {}
   const char *func_name() const { return "json_exists"; }
   bool is_bool_type() { return true; }
   void fix_length_and_dec();
@@ -85,8 +85,8 @@ class Item_func_json_value: public Item_str_func
   String tmp_js, tmp_path;
 
 public:
-  Item_func_json_value(THD *thd, Item *js, Item *path):
-    Item_str_func(thd, js, path) {}
+  Item_func_json_value(THD *thd, Item *js, Item *i_path):
+    Item_str_func(thd, js, i_path) {}
   const char *func_name() const { return "json_value"; }
   void fix_length_and_dec();
   String *val_str(String *);
@@ -99,8 +99,8 @@ class Item_func_json_value: public Item_str_func
 class Item_func_json_query: public Item_func_json_value
 {
 public:
-  Item_func_json_query(THD *thd, Item *js, Item *path):
-    Item_func_json_value(thd, js, path) {}
+  Item_func_json_query(THD *thd, Item *js, Item *i_path):
+    Item_func_json_value(thd, js, i_path) {}
   const char *func_name() const { return "json_query"; }
   bool check_and_get_value(json_engine_t *je, String *res, int *error);
   Item *get_copy(THD *thd, MEM_ROOT *mem_root)
@@ -291,12 +291,14 @@ class Item_func_json_merge: public Item_func_json_array
 class Item_func_json_length: public Item_int_func
 {
 protected:
+  json_path_with_flags path;
   String tmp_js;
   String tmp_path;
 public:
   Item_func_json_length(THD *thd, List<Item> &list):
     Item_int_func(thd, list) {}
   const char *func_name() const { return "json_length"; }
+  void fix_length_and_dec();
   longlong val_int();
   Item *get_copy(THD *thd, MEM_ROOT *mem_root)
   { return get_item_copy<Item_func_json_length>(thd, mem_root, this); }